GPS Reference Networks Need Regulation
by Harry O. Ward , PE
July 1, 2010
Contractors using GPS rovers or automated machine guidance are
making heavy use of GPS reference networks these days, and they are absorbing
3D data from satellites routinely. While doing research for an upcoming
article, I came across information that may call into question the accuracies
of GPS reference networks. I believe these potential faults should be
researched, evaluated, solved, standardized and regulated.
When I first wrote about GPS reference systems in January
2005 I initially referred to them as VRS for Virtual Reference Systems until I
found that Trimble had trademarked this term. Because of that, I titled my
article “Imaginary Reference Systems,” since the purpose was to create an
imaginary base station where the rover is. This article produced a number of
questions about the networks, such as “Who is watching the watcher?”
Why is that question important, you ask? It’s important
because we need to learn a lot more about how the systems actually work. The
manufacturers of the GPS systems developed the software that assembles the data
from surrounding base stations and performs the computations for dissemination
to the user. These manufacturers implemented mathematical formulas to read the
data, assess errors, perform corrections and apply those values to a dynamic
stream of data. The problem is that the manufacturers consider their systems to
be proprietary and therefore, are not open to scrutiny by the public.
When I wrote the article in 2005, I noted that each
manufacturer had developed software to produce corrections to the data being
captured. Trimble uses the Trimble RTKnet software; Leica developed Spidernet;
and Topcon offers the TopNet Reference Station Software Suite.
Based on e-mail responses I received from the article, many
people questioned the output of these solutions. So I decided to research the
issue and write a follow-up article on what algorithms were being used to
produce these computations.
As part of my research, I contacted each of the major
manufacturers of these systems and asked if we could discuss the mathematics. My
intent was to briefly describe how the corrections were being performed and
whether or not they were approved by a licensed surveyor. The meetings did not
go quite as I had hoped. Each manufacturer deferred the answers to my questions
by claiming that the systems were proprietary and not completely open to public
scrutiny. One advised me to read the white papers on the manufacturer’s website,
because that was the only public information they were willing to provide. I
did review those white papers, albeit briefly, yet they did not provide me with
the answers to my questions.
Left at a dead end in my research, I made the comment that
this is the first time that I could think of that a construction surveyor went
to sleep at night completely trusting in someone else’s answers. All of these professionals
spend a lot of time checking their answers and then they check the check. With
the advent of reference networks, contractors must trust the data they are
collecting even though they don’t know exactly where it came from.
This lack of information has left me with many questions
regarding these systems, including:
1)
Does a licensed professional oversee these
systems?
2)
What happens when a system crosses state lines,
was that professional licensed by the states in question?
3)
What happens when a construction surveyor
collects some data from a network and then for whatever reason, switches to
another network?
4)
Can one replicate exactly the same solution when
this switch occurs? If not, why not? Is that OK with everyone?
5)
Who is overseeing this national but independent
network of solutions?
Other related questions I have include: Who checks the algorithms
of these correction systems? Who verifies the inputs and outputs and the
dissemination of this data? Is the software bug free and if not, what
corrections are made and when are they made, after complaints or after QA/QC
occurs? Is QA/QC occurring? Are there licensed surveyor(s) overseeing,
reviewing and approving this information, the mathematics and the formulas on
which they are based?
Let’s say that a licensed surveyor does approve these
computations and processes. Well, what happens when a customer crosses into
areas where the approving licensed surveying professional is not certified or
licensed? This could easily occur if a project’s limits cross jurisdictions,
such as with the Heartland Corridor project that runs between Newport
News, Va., and Chicago. In that case, the contractor is
collecting information from multiple states networks. If there was a licensed
professional certifying the system, were they licensed by all of the states that
the systems serve? So where does the buck stop—who is responsible?
Now add to that another complication, one where the construction
surveyor jumps from one network to another manufacturer’s network. It is
possible that a project could begin using one network and later, due to a
weaker signal perhaps, the equipment operator elects to take advantage of his subscription
to an overlapping network. So when the collection of data must result in a
homogenous and seamless data stream, are the different manufacturers using
precisely the same algorithms? Have they implemented the mathematics in the
same way? I suggest that the answer is uniformly “No.” I say no because there
is currently no regulation, and since the manufacturers won’t divulge their
secrets, how could they possibly all be done the same?
Because of this uncertainty, I believe we need an
independent third party—perhaps the NGS (http://www.ngs.noaa.gov/)
or Weights and Measures (http://ts.nist.gov/WeightsAndMeasures/index.cfm)—to
step in, identify the issues involved, document the solutions and check to see
that the manufacturers have correctly implemented the solutions.
I recently read an article about the aeronautical industry
discussing a similar concept for airplanes trying to obtain guidance
information for landing aircraft. The author also seemed distressed that
regulation and conformity didn’t exist. The author recommended that all station
operators should standardize using the Radio Technical Commission for Maritime
Services (RTCM) Special Committee 104 standard for Differential Global
Navigation Satellite System (DGNSS) services. In another article[[1]],
the solution was that the “format of GPS measurement corrections should be
standardized to ensure that the system is independent of any single receiver manufacturer.
This can be solved by adopting the RTCM standard for RTK multiple reference
stations v3.0.”
I now pose this question to the practicing professionals,
those using the reference networks on a daily basis. Do you know the basis and
final implementation of the system you are using? Do you cross networks in
performing projects? How comfortable are you that you will get the exact same
answer from one position being collected by two or more reference networks? If
you cross state lines, who checked the computations you are collecting? Please
feel free to respond to this blog or e-mail me and let me know what your
thoughts are.
[1] www.mycoordinates.org/multiple-reference.php
Harry O. Ward , PE
hward@harken-reidar.com
Harry
O. Ward, PE, is president of Harken-Reidar Inc. He has been a member of the
engineering faculty at George
Mason University
since 1997 and was named “Outstanding Adjunct Professor” for GMU in 2010. He
can be reached at hward@harken-reidar.com or hward@gmu.edu.
Comments (1)Post a Comment
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Title: More Government Regulation - Not
By: GPSUSER
Posted: July 9, 2010 8:43 AM
Please.... Do we really need to solve this issue with more government? What we need is oversight, not regulation. As most states regulations already read, the responsibility is in the hands of the user, as a professional. As long as a user works on a known network, and takes check shots to ensure betwork preformance, i believe the government needs to stay out of this.